Photoimageable elements, such as lithographic printing plates, circuit boards, photolabels, photoresists, overlay color proofing films, and the like, typically consist of a substrate on which is coated a layer of a photosensitive material. Depending on the application, this photosensitive material can be coated on metal or plastic, for example. This photosensitive material, which can be a photopolymer, oligomer, monomer, or mixture thereof, typically either undergoes a reaction or promotes a reaction upon exposure to radiation thereby forming an image. In negative-acting systems, the nonexposed portions of the layer of photosensitive material are removed to produce an image. In positive-acting systems, the exposed portions of the layer of photosensitive material are removed to produce an image.
Negative-acting photosensitive imaging systems, e.g., photopolymerizable or radiation-polymerizable systems, typically include a layer of a composition containing a photosensitive material such as an unsaturated monomer or oligomer, a photoinitiator that is activated by radiation, binder resins, and optional colorants and other additives. In the production of free-radical initiated photopolymerizable coatings, the photoimaging reactions are subject to quenching by triplet oxygen thereby adversely affecting the imaging process. To prevent such triplet oxygen quenching of radiation formed free-radicals, the photosensitive layer can be overcoated with a polymer that has a low oxygen permeability. Such polymers are typically transparent, film-forming polymers that do not disrupt the photosensitive layer. In the past, suitable overcoat polymers were those that were soluble only in organic solvents. This created significant environmental concerns with respect to air quality and disposal, for example. More recently, water soluble overcoat polymers have been used to avoid many of the environmental concerns. One particularly suitable water soluble polymer is polyvinyl alcohol.
Polyvinyl alcohol is currently one of the most commonly used overcoat polymers in negative-acting, free-radical initiated, photosensitive imaging systems because of its low oxygen permeability. However, most polyvinyl alcohols are water soluble and therefore moisture sensitive. They also typically produce very smooth surfaces. Thus, they can be subject to blocking, particularly when they come into contact with a second smooth surface under conditions of high humidity. Furthermore, polyvinyl alcohol overcoat layers are often soft and easily embossed or scratched during the photoimaging process, as well as during storage, handling, and transport. One method by which at least some of these problems have been overcome is through the use of organic polymeric beads such as polymethyl-methacrylate beads. See U.S. Pat. No. 5,258,261 (Heller). However, the beads do not overcome the additional problem of blushing under conditions of high humidity, which results from moisture passing through the polyvinyl alcohol overcoat layer to the underlying photosensitive layer causing it to turn hazy, i.e., blush.